Neurogenesis persists into adulthood in many vertebrates including humans. An understanding of the factors that control adult neuron addition, differentiation and survival may ultimately suggest mechanisms for brain repair. In adult warm-blooded vertebrates, the most widespread neuron production occurs in the avian telencephalon. In songbirds, many new projection neurons are inserted into the High Vocal Center (HVC) to become part of the pre-motor, efferent pathway necessary for the production of learned vocalizations. Thus, studies of the avian brain may address basic questions about the mechanisms permitting widespread neuron addition and, at the same time provide an exceptional opportunity to relate neuron addition in discrete brain regions to a well-characterized behavior-song. Deafening leads to song deterioration and the impact on song decreases with increasing bird age or vocal practice. Correlations between singing history, song stereotypy, motor program stability (defined by reliance of song structure on auditory feedback) and neuronal incorporation established during the prior period of support will be directly tested by manipulation of singing prior to deafening. To further explore the functional significance of adult neuronal replacement, selective manipulation of HVC stem cells will be required. Using retroviral methods, the relationship between birthplace and final destination for HVC cells and neurons destined elsewhere in the telencephalon will be mapped. This work will identify sites of HVC stem cells for future manipulation and provide a fate map of the topographical relationship between proliferative zones and the telencephalon more generally. The first comprehensive analysis of adult-formed vocal control neuron structure and electrophysiological properties will also be conducted using retroviral vectors. Morphological comparisons among adult-formed HVC neurons and new neurons dispersed throughout the telencephalon will provide a first appraisal of the adult avian brain's natural potential to produce and integrate multiple neuron classes. Functional properties of adult-formed HVC pre-motor neurons (visualized by retroviral labeling with green fluorescent protein) and sources of synaptic input will be identified by electrophysiological stimulation-recording work in tissue slices as a step toward understanding how these cells are integrated into the brain. Collectively, this work will test the relationship between cell replacement and song, provide basic information about the avian brain's natural potential for producing diverse cell types, and set the stage for future work aimed at testing the functional consequences of suppressing replacement on learned vocal behavior.